There is known the seismic isolation technique using sliding pendulum seismic isolators essentially comprising convex supports coupled with concave sliding surfaces. Such isolators are usually arranged between a superstructure such as, for example, a bridge or a building, and its foundations. In case of earthquakes, the isolators allow a movement of the superstructure with respect to the foundations, thus protecting its integrity.
As an effect of the sliding movement of the convex supports on the concave surfaces, the superstructure oscillates increasing and decreasing its potential energy according to the law of motion of the pendulum, whose natural period is defined by the radius of the concave surface. The radius of the concave surfaces is designed in order to optimize the natural period of the pendulum for the reduction of the seismic response of the superstructure. Moreover, a certain amount of energy is dissipated through the friction of the contact material with the concave surface, thus reducing more the seismic response of the superstructure.
An example of such isolators is given in U.S. Pat. No. 4,644,714, in the name of Earthquake Protection Systems Inc., which discloses a sliding pendulum seismic isolator provided with a lower sliding element fixed on a foundation and an upper element fixed to a superstructure. The lower sliding element has a top concave surface on which an intermediate element slides which has a bottom convex surface of a corresponding curvature. The upper portion of this intermediate element is provided with a convex spherical surface coupled with the upper element through a corresponding concave spherical seat. The contact between this convex spherical surface and the corresponding concave spherical seat enables the relative rotation between the upper element and the intermediate element, which is caused by the movement on the lower concave surface.
An improvement over the above-mentioned support is disclosed in patent application US 2006/0174555 also in the name of Earthquake Protection Systems Inc., which describes a sliding pendulum seismic isolator provided with a lower sliding element and an upper sliding element between which three intermediate elements are arranged that are capable of carrying out relative rotations during the movements of the lower and the upper portions caused by an earthquake. Thanks to this arrangement, the main concave surface described in U.S. Pat. No. 4,644,714 is divided into two concave surfaces, a lower one and an upper one, resulting in a great reduction, for the same horizontal movement, of the floor dimensions of the isolator.
However, the friction caused by the contact and the sliding movement of the intermediate elements with respect to each other causes significant problems to the isolator, which exhibits such parasitic moments against the rotation that they penalize its dynamic response.
Moreover, friction causes significant wear problems to the components of known isolators, which results in complex lubrication systems and in a rather limited service life of the isolators.
Therefore, the object of the present invention is to provide a sliding pendulum seismic isolator capable of overcoming such drawbacks.